1. Field of the Invention
The present invention relates to a fabrication method of an LCD (Liquid Crystal Display) panel and, particularly to a fabrication method of an LCD panel having spacers dispersed on a display area thereof, in which liquid crystal falling-drop method for filling a gap of the LCD panel with liquid crystal is utilized.
2. Description of the Prior Art
In general, an LCD panel has a structure composed of a pair of opposing transparent substrates adhered to each other by a frame-shaped seal formed on a periphery of a display area thereof and liquid crystal filling a region of a gap between the substrates, which is surrounded by the seal. Furthermore, the LCD panel structure includes spacers (referred to as “seal spacers”, hereinafter) mixed in the seal and spacers (referred to as “display area spacers”, hereinafter) arranged in the display area, for maintaining a cell gap at an appropriate value.
In order to fill the gap between the substrates with liquid crystal, the fabrication method of the LCD panel includes either the liquid crystal injection method or the liquid crystal falling-drop method. In either the liquid crystal method or the liquid crystal falling-drop method, the rubbing step of printing an alignment film (polyimide film) and rubbing the alignment film to form grooves for aligning liquid crystal molecules is included after the patterning steps of an insulating film, which is necessary according to a liquid crystal display system employed, transparent electrode films, switching elements and a color filter layer, etc., which are formed on the transparent substrates. The liquid crystal injection method and the liquid crystal falling-drop method are different from each other in the assembling step subsequent to the rubbing step. The liquid crystal injection method and the liquid crystal falling-drop method will be described respectively.
Describing the assembling of the LCD panel in the liquid crystal injection method, the seal in the form of a closed line having an opening portion is formed around the display area on either one of the transparent substrates by a drawing printing using a dispenser or a screen printing, etc. And, the display area spacers are arranged on either one of the transparent substrates by forming them thereon or dispersing them. Thereafter, one of the transparent substrates is stuck on the other and the seal is hardened after the gap between the transparent substrates is regulated to a desired value while externally pressing the substrates. In this step, it is necessary to provide a desired cell gap. Thereafter, liquid crystal is injected into the gap between the transparent substrates through the opening portion of the seal as a liquid crystal injection port by usual vacuum injection method. Thereafter, the liquid crystal injection port is sealed by filling a sealing resin therein. According to the vacuum injection method, the LCD panel having the liquid crystal injection port is put in a vacuum device and, after the gap of the LCD panel is evacuated, the liquid crystal injection port is dipped in a liquid crystal tank filled with liquid crystal and liquid crystal is injected into the gap of the LCD panel while gradually increasing external pressure.
On the other hand, in the liquid crystal falling-drop method, the LCD panel is assembled through the steps mentioned below. The seal in the form of a closed line is formed around the display area on either one of the transparent substrates by a drawing printing using a dispenser or a screen printing, etc. And, the display area spacers are arranged on the transparent substrate by forming them or dispersing them. Thereafter, a suitable amount of liquid crystal is dropped onto the display area of the transparent substrate. Thereafter, one of the transparent substrates is stuck on the other and adhered each other in a vacuum chamber. Then, the mutually adhered transparent substrates are left under atmospheric pressure, so that the transparent substrates are deformed by a pressure corresponding to a difference between atmospheric pressure and a negative pressure within the LCD panel to reduce the gap between the transparent substrates. The seal is hardened at a time when a desired cell gap is obtained by such deformation of the transparent substrates.
In either the liquid crystal injection method or the liquid crystal falling-drop method, thermosetting resin or ultraviolet ray setting resin is mainly used as a material of the seal. Further, spherical or circular pillar shaped spacers are used as the seal spacers or the dispersed display area spacers. The display area spacers provided by forming them means the circular pillar shaped spacers formed by patterning.
When a plurality of LCD panels are formed with using a pair of large area transparent substrates, the transparent substrates are cut apart to form incomplete LCD panels and complete LCD panels are obtained by attaching polarizers onto front and rear surfaces of the incomplete LCD panels, respectively, in the final step of either the liquid crystal injection method or the liquid crystal falling-drop method.
In the fabrication method of the LCD panel using the conventional liquid crystal falling-drop method, a problem occurs during a period from a time when the incomplete LCD panel including the mutually adhered transparent substrates is left in atmospheric condition to a time the seal is hardened.
The problem of the fabrication method of liquid crystal display using the liquid crystal falling-drop method will be described with reference to a TFT driven, color LCD panel as an example.
FIG. 1A to FIG. 1C show cross sections of a LCD panel 20 fabricated by the conventional method. The LCD panel 20 is the TFT driven, color LCD. As shown in FIG. 1A to FIG. 1C, the LCD panel 20 is constructed with, mainly, a TFT substrate 1, a CF (Color Filter) substrate 2 opposing to the TFT substrate 1, liquid crystal 3 provided between the TFT substrate 1 and the CF substrate 2, a seal 4 for mutually adhering the TFT substrate 1 and the CF substrate 2 and sealing the liquid crystal 3, seal spacers incorporated in the seal 4 and display area spacers 16 arranged on a display area surrounded by the seal on the TFT substrate 1. The CF substrate 2 is provided with red, green and blue filter layers 22. An initial average size of the display area spacers 16 in a cell gap direction is set to a value equal to an appropriate cell gap value d0, which is necessary to perform a liquid crystal display appropriately. When the display area spacer is spherical or circular pillar shaped, the size thereof in the cell gap direction corresponds to a diameter thereof or, when it is a square pillar shaped spacer, the size corresponds to a side length of a square cross section thereof.
FIG. 1A shows a cross section of an incomplete LCD panel formed by dropping liquid crystal 3 on the TFT substrate 1 having the seal 4 and the display area spacers 16 arranged thereon and adhering the transparent TFT substrate 1 and the transparent CF substrate 2 together in a vacuum chamber immediately after the incomplete LCD panel is put in atmospheric environment. In this state, an inside pressure of the incomplete LCD panel 20 is negative, that is, lower than atmospheric pressure, and liquid crystal 3 does not fill in the incomplete LCD panel 20 completely as yet, leaving an empty portion 7. Thereafter, the incomplete LCD panel 20 is deformed due to the difference between atmospheric pressure and negative pressure inside the panel 20, so that a distance between the TFT substrate 1 and the CF substrate 2 is reduced. Since, in FIG. 1A, the TFT substrate 1 is put on a horizontal plane, the CF substrate 2 put on the TFT substrate 1 is deformed. Liquid crystal 3 is pressed by this deformation of the CF substrate 2 and gradually spread radially outward in parallel to the plane, so that the empty portion 7 is reduced.
However, the deformation of the incomplete LCD panel proceeds in a center portion of the LCD panel at higher speed than that in a peripheral portion as shown in FIG. 1B because, although atmospheric pressure exerted on the CF substrate is uniform, viscosity of liquid crystal 3 is very small compared with viscosity of the seal 4. That is, a resistance of liquid crystal 3 against the CF substrate 2 is small compared with a resistance of liquid crystal 3 against the seal 4. Viscosity of liquid crystal 3 is in the order of 0.02 (Pa·s) and viscosity of an adhesive resin used as the seal 4 is in the order of several tens to several hundreds (Pa·s).
Since the deformation of the incomplete LCD panel 20 proceeds in the center portion thereof at higher speed than that of the peripheral portion thereof, an inner surface of the CF substrate 2 first reaches the display area spacers 16 arranged in the center portion. The deformation further continues and, when the CF substrate 2 presses the display area spacers 16, the deformation of the CF substrate 2 is restricted by the resistance of the display area spacers 16. FIG. 1B shows this state. In the state shown in FIG. 1B, the display area spacers 16 arranged in the center portion of the LCD panel is pressed down and, therefore, the cell gap in the center portion of the LCD panel is smaller than the appropriate cell gap d0. This is because the initial size of the display area spacer 16 before compressed is set to the value equal to the appropriate cell gap d0.
Furthermore, since, in the state shown in FIG. 1B, there is the empty portion 7 left and pressures inside and outside of the panel are not equilibrated, that is, the inner pressure of the panel is different from atmospheric pressure, the deformation still further proceeds. This deformation of the CF substrate 2 proceeds in only the peripheral portion since the progression of deformation of the center portion is restricted by the resistance of the display area spacers 16 against the TFT substrate 1 and the CF substrate 2.
Thereafter, the deformation of the peripheral portion of the LCD panel proceeds until the LCD panel 20 becomes in the state shown in FIG. 1C in which the progression of deformation of the LCD panel 20 is stopped. FIG. 1C shows the state in which the empty portion 7 in the LCD panel 20 disappears due to the progression of deformation in the peripheral portion of the CF substrate, so that the inner volume of the panel 20 becomes equal to the volume of liquid crystal 3, achieving the pressure equilibrium between the inside and the outside of the panel.
In the state shown in FIG. 1C, the inner surface of the CF substrate 2 is not in contact with the seal spacers 5. That is, the seal spacers 5 are not pinched between the TFT substrate 1 and the CF substrate 2 and their spacer function to maintain the gap between the TFT substrate 1 and the CF substrate 2 at the appropriate value is not achieved enough, so that the peripheral portion of the LCD panel does not become the same as the appropriate cell gap.
Therefore, the cell gap value d1 of the center portion of the LCD panel 20 is smaller than the appropriate cell gap d0 (d1<d0) and the cell gap value d3 of the peripheral portion thereof is lager than the appropriate cell gap d0 (d3>d0), so that, between the center portion and the peripheral portion of the LCD panel, there is a portion in which the appropriate cell gap value is obtained as shown in FIG. 1C.
The reason for the pressure equilibrium between inside and outside of the LCD panel and the stoppage of progression of deformation in the state in which the cell gap is not uniform as mentioned above is that the amount of liquid crystal 3 is set to a value equal to the inside volume of the LCD panel 20 in the state in which the cell gap is equal to the appropriate cell gap d0 throughout the LCD panel. That is, a portion of liquid crystal 3 in the center portion of the LCD panel, which is compressed due to the reduced cell gap d1, is pushed into the peripheral portion, so that the cell gap d3 in the peripheral portion of the LCD panel becomes larger than the appropriate cell gap d0.
The state shown in FIG. 1C is a metastable state and sustained for some time period. If the hardening of the seal 4 were suspended for a considerably long time, the LCD panel 20 could be deformed in a manner that the cell gap becomes uniform throughout the LCD panel 20 due to the resistance of the display area spacers 16 in the center portion of the LCD panel 20. However, when the seal 4 is left in unhardened state for a too long time period under atmospheric pressure, there may be a case where the seal 4 is broken because pressure is directly applied to the seal 4 from the time when the LCD panel 20 is put under atmospheric pressure. Therefore, a time period from the time when the LCD panel is put in atmospheric pressure to the hardening time of the seal 4 is preferably from several minutes to several tens minutes. For this reason, the seal 4 has to be hardened under the condition in which the cell gap in the center portion of the LCD panel is smaller than the appropriate cell gap and that in the peripheral portion is larger than the appropriate cell gap, as shown in FIG. 1C.
The pressure acting on the seal 4 will be described with FIG. 2 which is a plan view of the TFT substrate including a pair of areas, which finally become two LCD panels, after the printing of the seals 4 and the dropping of liquid crystal 3 are performed therefor. In order to enhance the pressing force of atmospheric pressure to the transparent substrates after the latters are adhered each other, there is a case where an arbitrary seal 8 surrounding the seals 4 is formed as shown in FIG. 2. The auxilliary seal 8 is used to form a vacuum space surrounding the seals 4 and maintaining the enhanced pressing force.
Since the inner pressure of the seals 4 is increased with the progression of the above-described deformation of the LCD panels, the increased inner pressure is exerted on inner peripheral faces of the seals 4.
When the arbitrary seal 8 is broken by atmospheric pressure, the latter pressure is exerted on the outer peripheral faces of the seals 4. According to the fabrication method without using the arbitrary seal 8, atmospheric pressure is, of course, exerted on the outer peripheral faces of the seals 4 from a time at which the panel is put under atmospheric pressure.
In the liquid crystal injection method, liquid crystal is injected to the space surrounded by the seal through the opening portion thereof after the LCD panel is assembled by adhering the transparent substrates each other and hardening the seal therebetween, as mentioned previously. Therefore, even when the cell gap is smaller than the appropriate cell gap at the assembling time, it is possible to restore the appropriate cell gap by the resilient force of the spacers in the liquid crystal injecting step, since the seal has the opening portion. On the other hand, in the liquid crystal falling-drop method, the cell gap is formed in the state where liquid crystal is sealed by the seal and, therefore, once the non-uniform cell gap is formed, it is difficult to restore the appropriate cell gap.
As described above, in the fabrication method of a LCD panel using the conventional liquid crystal falling-drop method, the cell gap of the LCD panel obtained becomes non-uniform since the cell gap in the center portion of the LCD panel is smaller and the cell gap in the peripheral portion is larger compared with the appropriate cell gap. Since, therefore, the display area spacers arranged in the peripheral portion of the LCD panel do not support the substrates, the LCD panel may be easily deformed by external pressure and/or its weight, so that the uniformity of cell gap of the LCD panel can not be obtained. Moreover, since liquid crystal expands or contracts by variation of temperature and the amount of change of the cell gap in the center portion and in the peripheral portion is different, the display condition in the center portion of the LCD panel and in the peripheral portion thereof becomes different.
As a result of these matters, the display quality of the LCD panel is degraded.